The goals of differentiation are laudable, but in recent years, many question whether it is really possible for a teacher to tailor instruction for 20 to 30 different students and whether it's desirable to differentiate by learning styles. Differentiation is just one factor in effective instruction. Supervisors who walk into a classroom looking for differentiation run the risk of missing the forest for the trees. Wouldn't it be better to ask two broader questions (variations on Rick DuFour's well-known catechism): What are students supposed to be learning? Are all students mastering it? Research tells us what will produce high levels of student learning: appropriate goals; a positive classroom climate and culture; instructional strategies that will best convey the content being taught; and, among other things, effective use of assessment data to fine-tune teaching and follow-up with students who haven't yet mastered what's been taught.